(i) Field of the Invention
The present invention relates to a sponge rubber stamp having open cell, and a process for producing the same.
(ii) Description of the Related Art
A conventional stamp has been prepared by first irradiating (exposing) an original plate of a photosetting resin with ultraviolet rays through a negative having a stamp surface pattern, washing the original plate with jetted water to form a relief stamp surface pattern thereon, drying it, exposing it again to form a prototype having the stamp surface pattern thereon, press-bonding a phenol resin or the like onto the prototype under heating to form a matrix, and then press-bonding a rubber stamp material onto this matrix under heating.
In recent years, as stamps which can continuously be used for stamping without using any ink pad at each time of stamping, there have been spread stamps in which a sponge rubber having numerous open cell is used as a stamp material.
As the materials for the sponge rubber stamp having the numerous open cell which have been used to manufacture this kind of stamp, the following materials are known:
a. a rubber material mixed only with a foaming agent, PA1 b. a rubber material mixed with a water-easily soluble salt powder, a curing agent and a filler without using any foaming agent, PA1 c. a rubber material mixed with a water-easily soluble salt powder, a foaming agent, a curing agent and a filler, and PA1 d. a combination of two of the above-mentioned a, b and c.
For example, Japanese Patent Publication No. 48-41936 discloses a method which comprises superposing sheets A and B upon each other, and then carrying out vulcanization to obtain a rubber stamp.
That is, a rubber material is blended with a vulcanizing agent, a filler and a high-temperature closed cell foaming agent, and sodium chloride and a sodium naphthalenesulfonate-formalin condensate are then added thereto, followed by mastication to form a sheet A. On the other hand, the above-mentioned rubber material is blended with a vulcanizing agent, a filler, a low-temperature open cell foaming agent, an intermediate-temperature closed cell foaming agent and a high-temperature closed cell foaming agent, and sodium chloride and a sodium naphthalenesulfonate-formalin condensate are then added thereto, followed by mastication to form a sheet B.
Next, these sheets A and B are closely superposed upon each other, and then placed in a mold having an inside bottom surface formed with relief letters. Afterward, a pressure of about 200 kg/cm.sup.2 is applied to the mold so as to press the sheet A against the relief letters, and vulcanization is then carried out at gradually rising temperatures. Next, the molded article is released from the mold, washed with water until sodium chloride or sodium sulfate and the sodium naphthalenesulfonate formalin-condensate have been removed, dehydrated, and then dried to prepare a rubber stamp.
Furthermore, Japanese Patent Publication No. 59-28193 discloses a rubber stamp obtained by treating the following sheets A and B in the same manner as described above. This sheet A is formed by adding sodium chloride, sodium sulfate or sodium nitrate of 200-350 mesh and soluble potato starch of up to 200 mesh to an unvulcanized rubber blend, and the above-mentioned sheet B is formed by adding sodium chloride, sodium sulfate or sodium nitrate of 32-100 mesh and soluble potato starch of 150-250 mesh to an unvulcanized rubber blend.
In these methods, the sheet A is brought into contact with the mold formed with the relief letters, and the sheets A and B are then press-bonded between hot plates to form the rubber stamp.
In the conventional methods, however, a long time is taken to obtain the prototype, the matrix and the sponge rubber stamp, and so production efficiency is low. In addition, there is the problem that the materials have high costs.
On the other hand, another method is known in which a rubber plate is sculptured by the utilization of a laser beam to form a stamp surface.
For example, this technique is disclosed in Japanese Patent Publication Nos. 56-27388, 56-40033, 59-31435 and 62-4231, and this sculpture technique has not been applied to the above-mentioned sponge rubber material.
The present inventors have tried to sculpture the above-mentioned sponge rubber by a laser treatment. However, when a rubber sheet which has previously been made porous is sculptured by the use of the laser, the stamp surface precision of letters or figures is poor. In the case that a rubber sheet which is not porous is used as the material for the laser treatment and it is made porous after the laser treatment, microcracks take place in the rubber sheet when the rubber sheet is detached after the laser treatment, so that the stamp surface is poor in durability.